The extraordinary properties of graphene attract interest because of graphene's electronic properties. It is known that graphene can be considered a giant macromolecule that can be modified in chemical reactions [Geim, A. K. Graphene: Status and prospects. Science 324, 1530-1534 (2009) and Ruoff, R. Calling all chemists. Nature Nanotechnol. 3, 10-11 (2008)]. Graphene's surface has been functionalised to a limited degree with various atoms and molecules [Schedin, F., Geim, A. K., Morozov, S. V. Hill, E. W., Blake, P., Katsnelson, M. I. & Novoselov, K. S. Detection of individual gas molecules adsorbed on graphene. Nature Mater. 6, 652-655 (2007); Park, S. & Ruoff, R. S. Chemical methods for the production of graphenes. Nature Nanotechnol. 4, 217-224 (2009); Liu, H., Ryu, S., Chen, Z., Steigerwald, M. L., Nuckolls, C. & Brus, L. E. Photochemical reactivity of graphene. J. Am. Chem. Soc. 131, 17099-17101 (2009); and Sharma, R., Baik, J. H., Perera, C. J. & Strano, M. S. Anomalously large reactivity of single graphene layers and edges toward electron transfer chemistries. Nano Lett. 10, 398-405 (2010)].
Graphene is a single layer honeycomb lattice of carbon atoms and has attracted great interest due to its thermal conductivity properties, current carrying properties and electronic properties. Because of its band-structure, single layer graphene is a zero-gap semiconductor. However, in many applications, such as in semi-conductor devices, it would be desirable to engineer graphene to introduce suitable properties such as band gap so as to use in semiconductor devices, and other integrated electronics.
This requires the opening of an energy gap Eg in graphene's gapless electronic spectrum to allow field-effect transistors with sufficiently low dissipation in the off state. There are extensive efforts underway to open such a gap by physical means that include the use of nanoribbons [Han, M. Y., Ozyilmaz, B. Zhang, Y. B. & Kim, P. Energy band-gap engineering of graphene nanoribbons. Phys. Rev. Lett. 98, 206805 (2007); and Li, X. L., Wang, X. R., Zhang, L., Lee, S. W. & Dai, H. J. Chemically derived, ultrasmooth graphene nanoribbon semiconductors. Science 319, 1229-1232 (2008)]. The use of quantum dots [Ponomarenko, L. A., Schedin, F., Katsnelson, M. I., Yang, R., Hill, E. W., Novoselov, K. S. & Geim, A. K. Chaotic dirac billiard in graphene quantum dots. Science 320, 356-358 (2008)] represents another approach. Similarly, the use of strain [Guinea, F., Katsnelson, M. I. & Geim, A. K. Energy gaps and a zero-field quantum Hall effect in graphene by strain engineering. Nature Phys. 6, 30-33 (2010)], double gating, etc. [Geim, A. K. & Novoselov, K. S. The rise of graphene. Nature Mater. 6, 183-191 (2007); Castro Neto, A. H., Guinea, F., Peres, N. M. R., Novoselov, K. S. & Geim, A. K. The electronic properties of graphene. Rev. Mod. Phys. 81, 109-162 (2009); and Geim, A. K. Graphene: Status and prospects. Science 324, 1530-1534 (2009)] all represent methodologies that have been tried for this purpose.
Currently, there are two known derivatives of graphene: namely, graphene oxide (GO) and graphane. GO is essentially a graphene sheet randomly decorated with hydroxyl and epoxy groups and obtained by exposure of graphite to liquid oxidizing agents [Park, S. & Ruoff, R. S. Chemical methods for the production of graphenes. Nature Nanotechnol. 4, 217-224 (2009) and Eda, C. & Chhowalla M. Chemically derived graphene oxide: towards large-area thin-film electronics and optoelectronics. Adv. Mater. 22, 1-24 (2010)]. On a microscopic level, GO appears inhomogeneous with a mixture of regions that are pristine and densely decorated. Graphane, on the other hand, is a stoichiometric derivative of graphene with a hydrogen atom attached to each carbon [Sofo, J. O. Chaudhari, A. S. & Barber, G. D. Graphane: A two-dimensional hydrocarbon. Phys. Rev. B 75, 153401 (2007)]. This material is, as might be expected, relatively unstable and thus of no practical value for electrons and other applications. It is known that graphene hydrogenated from either one or both sides rapidly looses H at moderate T. Obviously, this is a serious disadvantage and casts doubts that graphane could ever be used in applications where a stability is required.
There are a number of problems in functionalizing graphene. One problem is that the resulting product may contain defects or dislocations. Presence of defects or dislocations can lead to the inhomogeneity of the product and for, some applications, does not enable the required electronic structure to be obtained. Functionalized or partly functionalized material can be unstable and tends to lose the bound atoms from its surface. It is thus can be very difficult to functionalize the graphene in either a uniform or complete manner which can render the end product unstable and useless for commercial applications.
There is therefore a need to address these challenges with known materials and to provide reliable methods for producing a uniform and homogeneous graphene derivative that is stable and has good electronic properties.